Willemite

Abstract: The coloring efficiency in different ceramic glazes of the classical Co olivine blue pigment (Co2SiO4) was compared with those obtained with a Co-doped willemite (Co0.05Zn1.95SiO4), and with a magnesium-doped Co-Al spinel (Mg0.2Co0.8Al2O4). The fired pigments and enameled samples were characterized by XRD, UV-VIS-NIR spectroscopy, CIE-L*a*b* color-measurements, and by SEM/EDX. The Co-olivine and Co-willemite pigments dissolved to a higher extent in the molten glazes than the Co–Al spinel. The darker blue color of the Co-olivine probed to be mostly due to Co2+ ions incorporated in the glassy matrix. The Co-willemite composition (which only contained a 1.3 Co wt.%) developed the bluest color hue of all pigments in both double and single firing glazes, while the magnesium-doped Co–Al spinel was found the most appropriate blue pigment in the bulk coloration of fast-fired porcelainized stoneware.

Abstract: Introduction. The structure of willemite was proposed by Bragg & Zachariasen (1930) and refined by Chin'Hang, Simonov & Belov (1970), using intensities from Weissenberg scans (Mo radiation) assessed against a standard v/2 blackness scale. Chin'Hang, Simonov & Belov (1970) obtained an R value of 0.143 for 250 reflections by refining the atomic coordinates. Upon examination the Si tetrahedron showed marked irregularity and a rather unusual S i O bond length of 1.68 A. Moreover, we could not evaluate the optical properties of willemite from the structural data by means of the point-dipole theory, as is possible for the other orthosilicates (Pohl, Eck & Klaska, 1978). A more accurate structure refinement of willemite therefore seemed necessary. Crystals of willemite were prepared hydrothermally at 693 K and 500 bar. Typical shapes are prisms elongated along e. The color is yeUow-brown. A transparent crystal of approximate dimensions 0.076 x 0.092 × 0.126 mm was selected for single-crystal investigation (mounted on the c axis). The cell parameters (294 K) were derived by the least-squares method from high-angle data for 30 Cu K<~ reflections in the 20 range 82 to 136 ° . The room-temperature intensity data were measured according to the fivepoint procedure described by Hoppe (1969), on a Siemens off-line three-circle diffractometer, with Cu K~t radiation and a graphite monochromator. 692 independent reflections were measured for l > 0 and 20 < 143 °, of which 669 with I _> 3tr(I) were accepted as observed. Lp corrections were applied. The data were corrected for absorption by Burnham's (1961) method. To describe the shape of the crystal, account was taken of 14 plane faces. The structure was refined by fullTable 1. Final fractional positional coordinates (× 104)for willemite

Abstract: Manganese doped nanocrystalline willemite powder phosphors $Zn_{2-x}Mn_xSiO_4$ $(0.1 \leq x \leq 0.5)$ have been synthesized by a low-temperature initiated, self-propagating, gas producing solution combustion process. The phosphors have been characterized by using x-ray diffraction (XRD), energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), and photo luminescence (PL) spectroscopic techniques. The lattice parameters calculated from XRD confirm that Zn2–xMnxSiO4 has a rhombohedral space group R3H. The XRD patterns confirm that $Zn_{2-x}Mn_xSiO_4$ phosphor samples undergo a phase transformation from \beta -willemite to \alpha -willemite phase at 950 °C. The EPR spectra of Mn2+ ions exhibit resonance signals at g \cong 3.24 and g \cong 2.02, with a sextet hyperfine structure centered around g \cong 2.02. The EPR signals of Mn2+ give a clear indication of the presence of two different Mn2+ sites. The magnitude of the hyperfine splitting (A) indicates that the Mn2+ is in an ionic environment. The number of spins participating in resonance (N), the paramagnetic susceptibility (\chi ), and the zero-field splitting parameter (D) have been evaluated as function of x. It is interesting to observe that the variation of N with temperature obeys Boltzmann. The paramagnetic susceptibility is calculated from the EPR data at various temperatures and the Curie constant and Curie paramagnetic temperature was evaluated from the 1/ \chi versus T graph. The luminescence of Mn2+ ion in Zn2SiO4 shows a strong green emission peak around 520 nm from the synthesized phosphor particles under UV excitation (251 nm). The luminescence is assigned to a transition from the upper 4T1 6A1 ground state. The mechanism involved in the generation of a green emission has been explained in detail. The effect of Mn content on luminescence has also been studied.

Abstract: Willemite (zinc silicate) is the main zinc mineral in some carbonate-hosted ore deposits (e.g., Franklin, New Jersey; Vazante, Brazil; Beltana, South Australia; Kabwe, Zambia). Recent interest in these unconventional zinc deposits has increased because of high zinc grades that exceed 40 wt percent, relatively low environmental impact of ore processing owing to the lack of acid-generating sulfides in the waste, and advances in ore processing technologies. In the past, most metallogenic studies proposed formation of willemite deposits by supergene or hypogene alteration of preexisting sulfide deposits. However, recent data on the Vazante, Beltana, and Kabwe deposits indicate willemite crystallization at temperatures in excess of 150°C, raising the possibility of primary precipitation from hydrothermal fluids. We use numerical geochemical modeling to examine the formation of willemite under hydrothermal conditions. Activity-activity diagrams reveal that, in the presence of dissolved sulfur and quartz, willemite instead of sphalerite will precipitate under oxidizing (e.g., hematite-stable, sulfate-predominant) and alkaline (pH higher than K feldspar-muscovite-quartz) conditions. Willemite also becomes more stable, relative to sphalerite, at high temperature, and willemite can coexist with magnetite at 300°C. The stabilities and solubilities of sphalerite, willemite, smithsonite, hydrozincite, and zincite were calculated for wide ranges of temperature (25°–300°C), chloride concentration, dissolved sulfur and carbon concentrations, pH, quartz saturation, and oxidation potential. Plots of the solubility of the different minerals as a function of two variables (e.g., temperature and redox state; pH and redox state) allow us to predict the effects of changing chemical conditions, which in turn permits an estimate of the efficiency of particular precipitation processes. Cooling is an effective process for precipitating sphalerite but not willemite, whereas pH increase (e.g., by acidic fluids reacting with carbonates) is effective for precipitating willemite but not sphalerite. Dynamic geochemical models that simulate physicochemical processes are used to understand the formation of the Beltana willemite deposit in the Adelaide geosyncline of South Australia. This small, high grade deposit (850,000 t at 36% Zn) is hosted in dolomite of the Cambrian Ajax Limestone, next to a tectonic contact with the diapiric, halite-bearing clastic sediments of the Callanna Group. The orebody is associated with hematite alteration and is characterized by the total absence of sulfides; willemite is the only zinc ore mineral, and the arsenate hedyphane (Ca 2 Pb 3 [AsO 4 ] 3 Cl) is the main lead mineral. The model results show that willemite will precipitate in response to water-rock interaction and fluid mixing processes at temperatures above 120°C. The presence of arsenate in the hydrothermal fluid is likely to have been important at Beltana; in arsenate-absent models sulfate is reduced to sulfide by the precipitation of ferrous iron as hematite, resulting in the precipitation of sphalerite and galena. In contrast, in models including arsenate the reduction of sulfate to sulfide is inhibited and willemite is predicted to precipitate.